This invention relates to systems for reducing the effects of impulse noise in video signals processed in a television receiver and more particularly to circuitry for detecting the occurrence of the impulse noise and for generating a signal to initiate signal compensation.
Impulse noise in the video signal of a TV receiver is characterized by abrupt signal aberrations of relatively short duration. It is typically generated as interference produced by the arcing of electric motors, e.g., hair dryers, vacuum cleaners, etc., lightning, automobile ignition systems, etc. These systems generate RF noise signals which tend to interfere constructively with TV broadcast signals in the RF spectrum.
Impulse noise generally has a broad frequency spectrum and a wide dynamic range. In TV receivers the signal spectrum is essentially as broad as the frequency response chracteristics of the circuitry through which it has been processed. The amplitude range of the impulse noise often exceeds the dynamic range of the composite video signal. Typically the impulse noise occurs as signal spikes of short duration, e.g., less than 2-3 microseconds and at a rate which is lower than the TV line frequency with an almost non-existent line-to-line correlation.
Impulse noise detectors heretofore employed have typically consisted of circuitry for determining the presence of large noise spikes. The video signal amplitude is compared with a threshold value set at the nominal maximum value of the composite video signal, and any signal exceeding the threshold is presumed to be impulse noise. This type of system is ineffective for determining impulse noise occurrences with amplitudes within the dynamic range of the composite signal, which noise may produce undesirable results on the displayed image.
The TV signal, from image line (horizontal) to image line is highly redundant. Video signal at corresponding points on adjacent lines will nominally be of similar amplitude values unless an abrupt horizontal edge occurs in the picture. The abrupt edge will generally affect the video signal in a number of lines so that the line-to-line signal before the edge is highly redundant, and the line-to-line signal after the horizontal edge, though different, is also highly redundant. It is very unlikely, in the absence of noise, that the signal of one horizontal image line will be radically different from the signals of both the preceding and succeeding horizontal lines. The present inventors utilize the line-to-line signal redundancy to advantage to determine when impulse noise is present. By successively performing a point-by-point comparison of the signal amplitude of one image line with corresponding points of the preceding line and the succeeding line the general redundancy pattern of the current line is determined with respect to the succeeding and preceding line. When no such relationship exists it is highly probable that impulse noise has been detected.